Patient-specific prostheses having one or more contoured surfaces to match with one or more of: a bone surface surrounding the area in which the prosthesis will be secured; a corresponding articular bone surface; and a surface that has been resected to receive the prosthesis, are known. The contoured surface(s) may be determined pre-operatively, through techniques including the use of computer-assisted image methods based on three-dimensional images of the patient's anatomy reconstructed from MRI, CT, ultrasound, X-ray, or other three- or two-dimensional medical scans of the patient's anatomy, to ensure that the prosthesis fits closely to the existing bone surface once located in position during a surgical procedure.
In particular, prosthetic glenoid components having a generally concave external bearing surface, to match the articular surface of a patient's associated humeral head, are known. On the opposite side to the bearing surface, a glenoid-contacting surface has one or more fixation elements protruding from it for location into the glenoid cavity in order to secure the component to the scapula. To date, at least one relatively large fixation element has been provided approximately centrally, for location in a corresponding central region of the glenoid cavity and penetrating deep into the glenoid vault.
These are typically secured in position according to the following standard operating procedure:
1. A deltopectoral incision is made along the anterior edge of the deltoid muscle (although other approaches also possible: posterior, superior, minimally invasive, etc.);
2. A resulting lateral skin flap is mobilised;
3. The fascia is incised, and exposed veins, nerves and deltoid muscle are retracted;
4. The rotator cuff is split;
5. Good exposure of the humeral head is achieved through antero-superior dislocation of the joint. Sometimes, tenotomy is required when dislocation is difficult to achieve;
6. The humeral head is resected of to increase space for glenoid preparation;
7. A k-wire is inserted centrally as a guide for a reamer and a drill guide;
8. A size-dependent reamer is then used to prepare a bone tunnel to receive the central fixation element;
9. A drill guide is positioned over the k-wire and manually aligned;
10. A drill sleeve is inserted and a first anchoring hole is drilled;
11. A fixation peg is used to prevent guide rotation;
12. At least a second anchoring hole is secured;
13. The instruments are removed;
14. An appropriate trial glenoid component is selected and the correct sized glenoid is determined;
15. If it is a cemented procedure, the holes are filled with bone cement;
16. The glenoid is pushed/impacted into the prepared bone;
17. Wait for cement to settle.
An exemplary scapula 100 is shown in FIG. 1, zoomed in on the glenoid 102. As is well known, the glenoid 102 comprises a generally concave glenoid cavity 104 surrounded by a glenoid rim 106. Glenoids 102 come in a variety of different sizes and shapes, but are typically tear-drop shaped, as seen in FIG. 1.
Prosthetic glenoid components therefore likewise come in a variety of different sizes and with differing shapes to best suit a patient's needs. Anatomically-shaped components can be provided, to accurately match a particular patient's glenoid shape. Alternatively, components having an optimal shape to suit a wide range of different glenoid shapes can be provided. Suitable generic shapes include: pear-shaped, in which a lower portion 110 has a larger radius than an upper portion 112, along a common vertical axis, as seen in FIG. 5a for example; oval, with upper and lower portions 110′; 112′ sharing a common radius size, as seen in FIG. 5b for example; and tear-drop shaped, in which a lower portion 110″ has a larger radius than an upper portion 112″, but horizontally offset from it, forming an asymmetric shape as seen in FIG. 5c. 
Due to the relatively small size of prosthetic glenoid components (for example in comparison to larger prosthetic components as used to replace other articular surfaces, such as the humeral head, or articular surfaces in the knee or hip, for example) and lack of bone support due to the topography of the scapula and especially the glenoid region thereof, it has proven difficult to provide reliable, strong, long-term fixation of such prosthetic glenoid components to the underlying glenoid cavity. Moreover, these difficulties are exacerbated by the fact that the central fixation element acts as a pivot for non-central forces acting on the component, thereby inducing rocking forces that function as a loosening mechanism, weakening the fixation of the component to the underlying bone. Also, the central fixation element projects into a hollow portion of the bone, so typically requires reinforcement through a cemented fixation. Where there is a cemented fixation, the repeated rocking motions serve to fracture the cement and therefore loosen the attachment of the component to the bone. Also, in cementless applications, the relatively large micromotion induced between the implant and the bone results in a lack of osseointegration.
An object of the invention is therefore to provide a more secure attachment of a prosthetic glenoid component to the underlying glenoid bone surface.
Where a prosthetic glenoid component becomes loose, as described above, it needs to be replaced through revision surgery. Such revision surgery is difficult because of the damage caused to the glenoid cavity 104 by the loosening glenoid component and the fact that a larger fixation element is needed to make a secure connection. This can lead to a sink-hole effect during revision surgery, whereby the weak central portion of the glenoid cavity 104 is destroyed as the primary component is removed (bone attached to the primary component's central fixation element either by cement or by bone in-growth comes out together with the component) and thus loses structural viability for reliably securing the revision component. This is shown in FIG. 2, in which FIG. 2a shows a glenoid 102 to which has been secured a primary prosthetic glenoid component having a central keel 120, and FIG. 2b shows a resulting sink-hole 122; FIG. 2c shows an alternative form of known primary fixation, using a plurality of pegs 124, with a resulting sink-hole 126 being shown in FIG. 2d. 
Another object of the invention is therefore to provide a prosthetic glenoid component that allows for better revision procedures.
A further object of the invention is to provide a prosthetic glenoid component that is sufficiently small as to be implanted through minimally invasive procedures.